Date:	Mon, 9 Nov 2015 22:35:42 +0200 (EET)
From:	Julian Anastasov <ja@....bg>
To:	Neil Horman <nhorman@...driver.com>
cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH] inet: delay address promotion check until last request
 in message


	Hello,

On Sun, 8 Nov 2015, Neil Horman wrote:

> On Sat, Nov 07, 2015 at 01:49:25AM +0200, Julian Anastasov wrote:
> > 
> > 	flush can provide many parameters. As there is no
> > any kind of indication in the netlink message that all addresses
> > are removed, we can not avoid the promotion.
> > 
> This is true, but seems irrellevant to me.  A flush operation is a sequence of
> RTM_DELADDR operations in a one or more netlink packets.  The way my patch is
> written, if a set of DELADDR requests is interspersed with other non DELADDR
> requests, then we do a promotion check between each consecutive set of DELADDR
> requests.  As such, all that happens is that the promotion check happens
> possibly more often than needed.  Its not optimal, but not harmful either.

	It is harmful, you miss promotion for some of the
subnets, see below...

> > > +	 * Only check for address promotion when this is the last request
> > > +	 * in this netlink transaction.  It allows this operation to complete
> > > +	 * in O(n) time rather than O(n^2)
> > 
> > 	It is not correct to assume that one promotion per
> > transaction is enough. The promotion happens in every subnet,
> > it was not once per device.
> > 
> 
> I'm not sure I understand the relevance here.  All I'm doing is, in effect
> masking the promote_secondaries sysctl for an interface doing a flush operation.
> Its equivalent to doing this in user space:
> 
> echo 0 > /proc/sys/net/ipv4/conf/<ifc>/promote_secondaries
> A=`some arbitrary address in <ifc>`
> ip addr del <every addressin in <ifc> except A>
> echo 1 > /proc/sys/net/ipv4/conf/<ifc>/promote_secondaries
> ip addr del A
> 
> Can you please explain to me the use case in which delaying a promotion
> operation until we think we're done ('done' being defined by the above transition
> from a DELADDR operation to a non-DELADDR operation in a netlink packet)
> produces an outcome that differs from the expectation with this patch in place?

	Here is how we can miss promotion...

dev=eth1
ifconfig $dev up
ip addr add 1.2.3.4/24 dev $dev
ip addr add 1.2.3.4/16 dev $dev
ip addr add 1.2.3.44/24 dev $dev
ip addr add 1.2.3.44/16 dev $dev
echo 1 > /proc/sys/net/ipv4/conf/$dev/promote_secondaries
ip addr flush dev $dev to 1.2.3.4/30
ip -V
ip utility, iproute2-ss010824

What happens is a request to delete just primary addresses:
1.2.3.4/24 and 1.2.3.4/16. The /30 is chosen in such a way,
so that any repeating attempts to flush the secondary
addresses are avoided. As result, with your patch, only
1.2.3.44/16 is promoted (the last secondary), 1.2.3.44/24
which is first in the list of the secondaries, is not
promoted, it is removed. You can even use 1.2.3.4/24,
1.2.3.5/16, 1.2.3.44/24 and 1.2.3.55/16 in case the
equal IPs are not a good example.

The problem will be more visible if one builds netlink message
by hand containing DEL for different primaries.

Regards

--
Julian Anastasov <ja@....bg>
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